This invention relates to liposomal formulations of compounds such as drugs. More particularly this invention relates to methods of increasing the encapsulation of desired compounds in liposomal formulations and the methods of making them.
When phospholipids and many other amphipathic lipids are dispersed gently in an aqueous medium they swell, hydrate and spontaneously form multilamellar concentric bilayer vesicles with layers of aqueous media separating the lipid bilayers. These systems commonly are referred to as multilamellar liposomes or multilamellar vesicles (MLV) and usually have diameters of from 0.2 .mu.m to 5 .mu.m. Sonication of MLV results in the formation of small unilamellar vesicles (SUV) with diameters usually in the range of 20 to 100 nm, containing an aqueous solution in the core. Multivesicular liposomes (MVL) differ from multilamellar liposomes in the random, non-concentric arrangement of chambers within the liposome. Amphipathic lipids can form a variety of structures other than liposomes when dispersed in water, depending on the molar ratio of lipid to water, but at low ratios the liposome is the preferred structure.
The physical characteristics of liposomes generally depend on pH and ionic strength. They characteristically show low permeability to ionic and polar substances, but at certain temperatures can undergo a gel-liquid crystalline phase (or main phase) transition dependent upon the physical properties of the lipids used in their manufacture which markedly alters their permeability. The phase transition involves a change from a closely packed, ordered structure, known as the gel state, to a loosely packed, less-ordered structure, known as the liquid crystalline state.
Various types of lipids differing in chain length, saturation, and head group have been used in liposomal drug formulations for years, including the unilamellar, multilamellar, and multivesicular liposomes mentioned above. One of the major goals of the field is to develop liposomal formulations for sustained release of drugs and other compounds of interest, and liposomal formulations from which the rate of release of the encapsulated compound can be controlled.
These goals are important and many studies have been undertaken towards achieving them. Another less recognized goal, increasing the yield of product from a liposomal formulation used as a delivery agent, has very practical benefits as well, particularly to the pharmaceutical industry. For instance, increasing the percent of drug encapsulated in liposomal formulations can result in increased yield and substantial cost savings. In the case of liposomal drug formulations, it is also desirable to have the highest possible percent of drug encapsulated for any given lipid:drug ratio to avoid the need for injecting highly viscous formulations or large volumes into the patient in order to achieve a desired dosage. If a process results in a high percentage of compound encapsulated but yields a product with a low drug:lipid ratio, it is generally necessary that the formulation have a high lipocrit (analogous to hematocrit) in order to satisfy a specified drug dose, or provide a therapeutically effective amount of a biologically active substance via an injection. Analogous to hematocrit, lipocrit is a measure of the percent volume occupied by the liposomes relative to the total volume of the liposome suspension. Yet, such formulations are difficult to administer by injection because of their high viscosities. Thus, the need exists for more and better methods for obtaining liposomal formulations that maximize the efficiency of a drug encapsulation to achieve a low lipid:drug ratio.